MEMS microphones usually comprise a conductive backplate and a conductive, flexible membrane arranged at a distance from the backplate. The backplate and the membrane realize electrodes of a capacitor. When a bias voltage is applied to the electrodes, oscillations of the membrane caused by received acoustic signals are converted into electrical signals. For further signal processing MEMS-microphones can comprise a ASIC (Application-specific Integrated Circuit) chip.
The microphone has a central region and a rim region. The central region is defined as the acoustically active region of the microphone. This central region is surrounded by the rim region that is acoustically inactive. Within the rim region, connection means for mechanically and/or electrically connecting the backplate and the membrane are arranged on a substrate material. For constructing MEMS microphones, manufacturing processes of semiconductor devices such as layer deposition, deposition of photo resist films, structuring photoresist films, and partly removing structured layers are utilized. The capacitor has an acoustically active section being defined by the acoustically active region of the microphone. Further, the capacitor has an acoustically inactive section being defined by the capacitor's section within the rim region.
A MEMS microphone comprises a system of stacked layers. However, the capacitor's acoustically inactive section deteriorates the performance of the microphone due to parasitic capacitance. The corresponding signal attenuation is:Hc=Cm/(Cm+Ci+Cp),  (1)where Ci is the input capacitance of an according ASIC chip processing the electrical signal, and Cp is the parasitic capacitance, i.e., the capacitance of the capacitor's acoustically inactive section. Cm is the total capacitance comprising the capacitance of the rim region and the capacitance of the central region. It can be seen that reducing the parasitic capacitance reduces the signal's deterioration and, thus, improves the microphone's signal quality.
Cp mainly depends on the acoustically inactive overlapping region of the backplate and the membrane.